generic type and typed function that conditionally extend an object if the keys / value type of the first object are not in the second

XorExtend.ts

type XorExtend<T extends {
    [K in keyof T]: K extends keyof A
    ? A[K] extends T[K]
        ? never
        : T[K]
    : T[K]
}, A extends object,> = T & A

declare function xorExtend<O, P extends { [K in keyof P]: K extends keyof O
    ? O[K] extends P[K]
        ? never
        : P[K]
    : P[K]
}>(a: O, b: P): P & O